Two recently identified transient receptor potential (TRP) channels, TRPM8 and TRPA1, have been proposed to play an important role in mammalian cool and cold peripheral sensory transduction. When expressed in cell-lines the cloned TRPM8 and TRPA1 receptors have distinct pharmacological and temperature response characteristics. Although these receptors are also transported to the central terminals of primary afferents, little is known about their centrally mediated actions. In this thesis, I use an in vitro electrophysiological approach to investigate the dorsal horn processing of cool afferent modalities and the role of TRP ion channels. The results of this thesis provide further information on thermal processing, indicate direction for further research and suggest possible therapeutic targets for the management of abnormal cold sensory processing.
Initial experiments demonstrate that the cooling agents and known TRPM8 and TRPA1 agonists, menthol and icilin, inhibit primary afferent evoked excitatory postsynaptic currents (EPSCs) in rat spinal cord dorsal horn neurons. In addition, temperature reduction, menthol and icilin increase the frequency of miniature EPSCs without affecting amplitude distribution or kinetics. Little or no direct postsynaptic effect on dorsal horn neurons, GABAergic or glycinergic transmission was found. In combination, these observations demonstrate that temperature reduction, menthol and icilin act presynaptically to increase the probability of glutamate release from primary afferent fibres.
Further examination of the changes in glutamatergic synaptic transmission induced by temperature reduction, menthol and icilin reveals a subset of neurons sensitive to innocuous cool (< 29 oC) and low concentrations of icilin (3-10 µM) which closely match the temperature activation and pharmacological profile of TRPM8. In addition, the majority of lamina I and II neurons displayed characteristics partly consistent with TRPA1-activation, including a concentration-dependent response to icilin and blockade by ruthenium red. The present experiments did not allow thermal characterisation of these TRPA1-like responses. Together these observations indicate that the effects of menthol and icilin on glutamatergic synaptic transmission in the superficial dorsal horn are mediated by TRPM8 and possibly by TRPA1.
Examination of the anatomical location of neurons activated by temperature reduction, menthol, icilin and capsaicin allowed the central termination pattern of thermoreceptive primary afferent fibres with specific TRP-like response characteristics to be determined. TRPM8-like presynaptic activation was confined to a subpopulation of neurons located in lamina I and outer lamina II, while the majority of neurons throughout laminae I and II received inputs sensitive to menthol, high concentrations of icilin and capsaicin. These findings suggest that innocuous cool sensation projects to a specific subpopulation of superficial dorsal horn neurons unlike other modalities (mediated by TRPV1, possibly TRPA1 and other receptors), which non-selectively engage circuits within the entire superficial dorsal horn. No morphological specificity was identified for recovered neurons after electrophysiological characterisation.
Finally, mu-opioids were shown to inhibit basal glutamatergic synaptic transmission as well as menthol- and icilin-induced transmission in the superficial dorsal horn. Of particular interest, delta-opioids selectively inhibited icilin-induced synaptic transmission within the same location. The selective effect of delta-opioids suggests a possible role in modulating receptors activated by icilin (TRPM8 and TRPA1).
Overall, this thesis provides further evidence that TRPM8 is responsible for the transduction of innocuous cold sensation in mammals and is a potential therapeutic target in humans with cold hyperaesthesia secondary to abnormal thermal processing. The use of delta-opioid agonists warrants further investigation in cold hypersensitivity states and potentially other forms of pain.